Microelectronic devices generally comprise a thin slab of a semiconductor material, such as silicon or gallium arsenide, commonly called a die or a semiconductor chip. In one face of the die is fabricated the active circuitry. To facilitate electrical connection to the active circuitry, the die is provided with bond pads on the same face. The bond pads are typically placed in a regular array either around the edges of the die or, for many memory devices, in the die center. The bond pads are generally made of a conductive metal, such as gold or aluminium, around 0.5 μm thick. The size of the bond pads will vary with the device type but will typically measure tens to hundreds of microns on a side.
Wire bonding and flip-chip interconnection are two schemes used for making contact to the die bond pads. In wire bonding, the die is attached to a substrate in a face-upwards orientation and fine wire is connected to each bond pad by a solid state joining method such as ultrasonic welding or thermo-compression diffusion bonding. In flip-chip interconnection, lumps of metal are placed on each bond pad. The die is then inverted so the metal lumps provide both the electrical pathway between the bond pads and the substrate as well as the mechanical attachment of the die to the substrate. There are many variations of the flip-chip process, but one common configuration is to use solder for the lumps of metal and fusion of the solder as the method of fastening it to the bond pads and the substrate. When it melts the solder flows to form truncated spheres. Depending on the dimensions of the solder sphere this is referred to as a ball grid array (BGA) interface or a micro ball grid array (μBGA) interface.
Semiconductor devices used as image sensors usually require a face-up orientation such that the scene of interest can be focused (or projected) on the active circuitry. For commercial reasons, it is often desirable that the die are connected to the substrate using a BGA or μBGA interface.
One approach to connect the die bond pads on the front face of the die to a BGA interface on the rear face of the die is to provide wiring traces that extends from the die bond pads over the front face of the die, down the sides of the die and onto the rear face of the die. This type of lead contact is often referred to as a “T-style contact” because the wiring trace on the edge of the die and the wiring trace on the front face of the die appears to form a “T” where they join. FIGS. 2a and 2b illustrate an example of a T-style contact.
FIG. 2a shows a schematic frontal 200 view and FIG. 2b a cross-sectional view 250 of a single T-style contact of a semiconductor package. The die is drawn upside down so the front face 201/251 is towards the bottom of the page and the rear face 202/252 towards the top of the page. A bond pad 203/253 on the front face connects to the wiring trace 204/254 on the edge of the die. The wiring trace continues to a land 205/255 on the rear face, where it joins to a solder sphere 206/256. The shape of the T-style contact 257 is evident in the cross-sectional view, while the sidewall angle 207 is indicated in the frontal view. The drawing is not to scale.
An alternative approach for an image sensor package is to use through silicon vias (TSV) to connect the bond pads to the BGA interface. FIG. 3 is a cross-section view 300 of a typical TSV. The TSV is a hole (or blind via) extending through the thickness of the semiconductor that terminates on the underside of a bond pad 304. The sides or wall of the through hole are coated with metal to form an electrically conductive pathway between the front and rear surfaces of the die. A deep reactive ion etching process known in the engineering community as the ‘Bosch process’ may be used to form the TSV shown in FIG. 3. The contact to the bond pad 304 shown in FIG. 3 is often described as U-style. To complete the electrical circuit between the underside of the die bond pads and the conductive coating applied to the walls of TSVs requires a solid state bond between the two metals.
FIG. 3 shows the semiconductor die inverted with a front face 301 and a rear face 302. A hole 310 extends through the thickness of the die and the dielectric film 303 underneath the bond pad 304 to terminate on the bond pad 304. A dielectric material 311 and a conductive coating 312 line the walls of the hole. Both the dielectric material lining the through hole 310 and the conductive coating 312 extend on to an area of the rear face 302 of the die. Hole 310 extending through the silicon is parallel-sided and perpendicular to the die faces 301 and 302.